The invention relates to a self-optimizing channel equalization and detection method comprising receiving a signal, taking samples of the signal within each symbol period over a timeslot, calculating reference constellation points within each symbol period on the basis of a channel estimate, updating the channel estimate within each symbol period on the basis of an error between each sample point and the reference constellation point, the error having been processed by one or more adaptivity parameters, and defining; for bit detection, the signal path having the best error metrics in the timeslot on the basis of the error metrics calculated from the sample points on the basis of the channel estimate.
When information is transferred on a radio channel, the signal to be transmitted has to be subjected to modulation. Modulation converts the signal into a form in which it can be transmitted at radio frequency. A modulation method can be considered efficient for instance if it allows as much information as possible to be transferred at as narrow a frequency band as possible. Depending on the purpose of use, other features can also be emphasized. Modulation should also cause as little interference as possible to adjacent channels.
Modulation methods include e.g. xcfx80/4-DQPSK (xcfx80/4-shifted, Differential Quaternary Phase Shift Keying) modulation. This modulation method comprises eight phase states, but only four phase shifts. Allowed phase shifts (symbols) are xc2x1xcfx80/4 and xc2x13xcfx80/4. FIG. 3A shows the modulation phase shift diagram (constellation). Each phase shift corresponds to two bits to be transmitted. In other words, a digital signal modulates the carrier in two-bit periods so that a given phase shift during each symbol period corresponds to a given two-bit combination. A symbol period refers to a signal period employed in the transmission of two bits. Phase shifts corresponding to bit combinations 00, 01, 10 and 11 are xcfx80/4, 3xcfx80/4, xe2x88x92xcfx80/4 and xe2x88x923xcfx80/4. The symbol frequency used in e.g. the TETRA system (Terrestrial Trunked Radio) is 18 kHz. the bit frequency being 36 kHz.
When a signal is being received, it has to be demodulated in order for the information therein to be detected. However, a signal transferred over the radio path can be distorted in various ways, thus complicating modulation detection. Signal-impairing phenomena include e.g. noise and inter-symbol interference (ISI). A signal-distorting phenomenon also arises when a signal on a radio connection is reflected from various obstacles, such as buildings and irregularities in the terrain. In this case, the signal detected at a receiver is the sum of a plurality of propagation paths. Each propagation path is different in length and signals arrive at the receiver at different points of time, i.e. the delay varies. In addition, the movement of a vehicle causes frequency deviations in relation to speed, the deviations being called Doppler frequencies.
To correct signal distortions upon reception of a signal, various channel models for describing the signal-distorting properties of a channel are used in a receiver. In fact, a channel equalizer in a receiver uses such channel models to equalize channel-induced distortions. In other words, the channel equalizer acts as a kind of a filter. In the TETRA system, channel models include e.g. AWGN. RAx, TUx and HTx. AWGN is a static channel describing e.g. the connection between a stationary terminal and a base station in conditions not involving signal reflection. RAx refers to conditions in a rural area: flat terrain without reflections. X describes the speed of motion of a terminal. TUx refers to a typical urban environment having a relatively weak second beam which is reflected at a small delay. A typical speed in urban conditions is 50 km/h, and consequently a channel model TU50 is used to describe urban conditions. HTx., in turn, refers to conditions in a hilly terrain having a strong second beam which is reflected at a fairly long delay. The TETRA specifications define a channel model HT200, which consequently describes a vehicle moving in hilly terrain conditions at a speed of 200 km/h.
The problem in the above arrangement is that when a given channel model is used to optimize the channel equalizer of a receiver, the performance of the channel equalizer suffers in other types of conditions. The properties of radio channels typically vary continuously as a function of time, making pre-optimization of reception filtering impossible.
It is an object of the invention to provide a method for solving the above problems. The objects of the invention are achieved by a method which is characterized by updating one or more adaptivity parameters used in channel estimate update within each timeslot in a direction which tends to decrease the error metrics of the best defined signal path within a single-timeslot or multiple-timeslot observation period when being compared with the error metrics of a corresponding path in a previous timeslot.
The invention is based on changing one or more adaptivity parameters used for channel estimate calculation in a way which allows the error metrics of the best signal path in the timeslot under observation to be minimized within a single-timeslot or multiple-timeslot observation period. This change ensures that an optimal reception is always achieved in varying channel conditions. The adaptivity parameter is a coefficient determining to what degree a channel estimate is changed on the basis of the error between the reference constellation and the actual sample. The higher the adaptivity parameter. the faster the resultant adaptation, which is preferable in rapidly changing channel conditions. On the other hand a low adaptivity parameter value typically results in better noise tolerance. In accordance with a preferred embodiment of the invention, a constant is added to or subtracted from the channel estimate adaptivity parameter in such a way that the change in the error metrics caused by the previous adaptivity parameter update causes the error metrics to decrease as a result of the addition/subtraction of the constant.
It is an advantage of the method of the invention that the same channel equalizer can be used in different types of channel conditions so that the operation of the channel equalizer is always optimized in accordance with the current channel.
The invention also relates to a self-optimizing channel equalizer/detector, which is adapted to receive signal samples within each symbol period over a timeslot, calculate reference constellation points within each symbol period on the basis of a channel estimate, update the channel estimate within each symbol period on the basis of an error between each sample point and reference constellation point, the error having been processed by one or more adaptivity parameters, define the signal path having the best error metrics in the timeslot on the basis of the error metrics calculated from the sample points on the basis of the channel estimate, and detect the bits corresponding to the signal path. whereby the channel equalizer/detector is characterized by being adapted to update one or more adaptivity parameters used in channel estimate update by timeslots in a direction which tends to decrease the error metrics of the best defined signal path within a single-timeslot or multiple-timeslot observation period, when being compared with the error metrics of a corresponding path in a previous timeslot. Such a channel equalizer/detector allows the advantages of the method of the invention to be achieved with a simple structure.